This invention relates to silylation reagents which are suitable for the preparation of binders carrying silanyl groups in the side chain and soluble in aqueous alkali.
In the submicron region, in particular, with increasing resolution and at the same time decreasing focussing tolerance for structuring substrates with complicated topography, microlithography requires the use of multilayer systems. The two-layer system is preferred because of simpler processability.
Usually the substrate has a complicated topography which is due to process steps which are standard in the manufacture of microelectronic circuits such as, for example, metallization, oxide deposition, epitaxy etc.
In the two-layer process, the topographically uneven substrate is leveled by a so-called planarization layer. As materials for the planarization layer, use is typically made of polymers which can be satisfactorily etched in the subsequent plasma etching process.
The actual resist layer, which is exposed through an image, is applied to the planarization layer. After the development of the exposed or unexposed regions (depending on the process used, i.e., positive- or negative-working), the resist layer which has not been removed acts as a mask for plasma etching the planarization layer thereunder which is now not covered by the image. The resist layer must therefore be resistant to plasma etching, in particular to oxygen plasmas, and at the same time must make highly resolved structures possible. The preparation of highly resolved structures by means of multi-layer processes, in particular two-layer processes, is described in detail by E. Reichmanis, G. Smolinsky and C. W. Wilkins in Solid State Technology, August 1985, page 130. Photosensitive layers on planarization layers are reviewed by Y. Onishi et al. in SPIE, vol. 539, Advances in Resist Technology and Processing II, page 62.
The resistance of binders to plasma etching, in particular the resistance to oxygen plasmas, can be achieved by incorporating metal atoms (for example Si, Ge, Sn, Ti). In practice, the use of silicon has proved successful: silicon-containing compounds are relatively readily accessible synthetically, have in general a low toxicity and do not represent a source of contamination in processes for manufacturing microelectronic circuits. Hitherto known silicon-containing binders which contain polysiloxane groups bound to conventional binders are described in U.S. Pat. No. 4,481,049.
Binders of this type, however, have too low a glass transition temperature so that it is not possible to prevent the resist structures flowing under thermal load, such as may occur, for example, in plasma etching processes. As a consequence thereof, it is not possible to prevent a loss of dimensional stability. Binders of this type are also incompatible with other binders, in particular with alkali-soluble binders. In addition, their low lithographic sensitivity and the restriction to irradiation wavelengths in the deep UV light range are disadvantageous.
A polysiloxane which contains disilane units is described by M. Ishikawa et al. in J. Polym. Sci. 21, page 657 (1983). Added to the known low glass transition temperature of polymers containing Si--O--Si groups is, in the case of this compound, also an elaborate synthesis of the silicon-containing monomers necessary for these compounds.
Furthermore, polymers are known which have polysilane groups as a constituent of the main chain and which contain two to six Si atoms linked to each other (EP-A-0,129,834, JP-A-153931/1983, JP-A-061831/1984, JP-A-008839/1985, JP-A-118745/1986, JP-A-189533/1986 and JP-A-025744/1987). Although compounds of this type are resistant to reactive oxygen-ion etching, they are at the same time active as chromophores towards deep UV light, i.e., they can be cleaved by light of these wavelengths. This makes them disadvantageous for use in photosensitive mixtures: the irradiation wavelengths are restricted to the deep UV region and are consequently not usable in the current I- and H-line exposure equipment employed for mass production. A further disadvantage is the difficulty of synthesizing these compounds, since alkali-metals have to be used in a highly reactive form. At the same time, there is no immediate guarantee that polymers will be produced which are soluble in aqueous alkali. This would entail development of a solvent, which is not considered feasible due to environmental protection standards. In addition, they are not compatible with cresol-formaldehyde-novolak resins since mixtures of these components immediately separate before they are able to form a uniform film on the base (substrate), or on the planarization layer.
Binders for photosensitive mixtures which contain monosilane groups (--SiR.sub.3) are described in EP-A-0,096,596. Disadvantages of the compounds of this type are, inter alia, a complicated monomer and also polymer synthesis. By introducing only one silicon atom per monomer unit, it is possible to achieve the silicon content of the binders required as a minimum for an appropriate resistance to plasma etching only with difficulty, if at all. A serious disadvantage is that these binders permit only an image differentiation due to development with organic solvents.
DE-A-2,217,744 describes polymeric binders which also contain --SiR.sub.3 -- groups, but in this case are produced by a polymer-analogous reaction of polymeric binders with reactive silicon-containing compounds, or by copolymerization. Whether these compounds are resistant to plasma etching is not evident from this publication, but they are claimed to improve the adhesion to substrates. Due to the chemical reactivity of Si--X bonds, the storage life of these compounds is very limited and under certain circumstances a post-curing is possible.
Binders carrying methylene-linked Si fragments are described as constituents of resist systems in JP-A-177005/1987. These can, however, be prepared only by means of a multi-stage and elaborate synthesis.
Si-containing binders in which the phenolic hydroxyl functions are linked to silicon via ester or iminoester functions are described in JP-A-264342/1986. A disadvantage is, however, that the silicon-ester or silicon-iminoester units described may also be decomposed at the wavelengths used for the photolysis of the naphthoquinonediazides or benzoquinonediazides added, and are split off. A further disadvantage of the compounds cited is their chemical instability towards, for example, hydrolytic decomposition, which has an unfavorable effect on the storage stability. It is furthermore disadvantageous that the multi-stage synthesis of the compounds cited is difficult to reproduce and proceeds with low overall yields.
Binders to which monomers are added, containing disilanyl units to impart resistance to plasma etching to the host polymer, are described in U.S. Pat. Nos. 4,481,279 and 4,464,455, as are similar compounds in JP-A-125729/1984, 125730/1984 and 121042/1984. The disilanyl compounds added to the binder are anchored in the polymer matrix by ionizing radiation. Before the development step by means of reactive ion etching using oxygen, the disilane is removed from the unexposed region with the aid of a vacuum process, for example. The disadvantage of the process is, inter alia, that the differentiation in terms of image is unsatisfactory because silicon-containing monomers are still present in the unexposed regions and can only be removed inadequately, even by the above-mentioned vacuum process.
There presently exists a need for a silylation reagent which can be utilized successfully for preparing binders of high purity which will have the following properties: An adequate solubility in aqueous alkali, a very high resistance to plasma etching in the case of reactive ion etching with oxygen, a long storage stability, and a high resistance to radiation so that exposure to an image of photosensitive mixtures can be undertaken (I- and H-exposure equipment, wavelength greater than 300 nm).